1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor sheet carrying a radiation image stored thereon to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and detecting the emitted light to obtain electric image signals. This invention particularly relates to a radiation image read-out apparatus which efficiently guides and detects the light emitted by the stimulable phosphor sheet and which is formed small.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals, which are processed as desired to reproduce a visible image having an improved image quality, particularly a high diagnostic efficiency and accuracy.
In the aforesaid radiation image recording and reproducing system, detection of the light emitted by the stimulable phosphor sheet has heretofore been carried out in two manners as described below.
Specifically, picture element division is effected by scanning with stimulating rays, and the light emitted by the stimulable phosphor sheet is detected by a light receiving device having a wide light receiving face, such as a photomultiplier or a photoconductive device. Alternatively, picture element division is effected by a light receiving device such as a two-dimensional solid state image pickup device or a semiconductor line sensor, and electric image signals are sequentially generated by an electric circuit.
However, with the former method, it s necessary to use a light guide member having a complicated shape for efficiently guiding the light emitted by the stimulable phosphor sheet to the photodetector, and therefore the read-out apparatus becomes large. Specifically, as disclosed in, for example, U.S. Pat. No. 4,346,295, the aforesaid light guide member is obtained by forming one end of a light guiding sheet in a linear shape to face a main scanning line of stimulating rays on the stimulable phosphor sheet, and bending the other end of the light guiding sheet to match the light receiving face of the photodetector. The light guide member having such a complicated shape requires a large installation space.
On the other hand, in the case where picture element division is effected by a light receiving device, the light receiving face of each of the elements constituting the light receiving device becomes small, the efficiency of guiding of the light emitted by the stimulable phosphor sheet becomes low, and therefore the S/N ratio in the read-out image signals is deteriorated. To solve this problem, a condensing lens may be utilized for guiding the light emitted by the stimulable phosphor sheet. In this case, however, the light receiving solid angle of the lens cannot actually be made so large, and therefore the light guiding efficiency cannot be improved so much.
Also, as the light receiving area of the photodetector becomes large, the dark current increases, and the S/N ratio deteriorates with respect to a predetermined optical amount. This tendency becomes more pronounced in the case of a solid state semiconductor device. Therefore, it is desired to improve the light guiding efficiency without increasing the light receiving area.